Fix OS X build issues

These changes remedy issues I found building and running the Twine app with OS X Mavericks. Now I'm able to generate a working Twine.app that requires no other folders (in line with normal OS X application structure), and I can run the virgin app.py script if desired.

Verified the changes didn't break anything on Windows 7 -- ran app.py on its own, and ran the twine.exe file resulting from buildexe.py. Haven't tried with other versions of OS X or Windows.